1. Field of the Invention
The present invention relates to a technique of compressing and auto-igniting an air-fuel mixture of a fuel and the air in a combustion chamber and thereby taking out power. More specifically, the invention pertains to a technique of controlling auto ignition of the air-fuel mixture to take out power with a high efficiency, while effectively reducing emission of air pollutants through combustion.
2. Description of the Related Art
Internal combustion engines produce large power by relatively small-sized bodies and are thus widely used as power sources of diverse transportation devices like automobiles, ships, and aircraft as well as stationary power sources in factories. The operation principle of such internal combustion engines makes a fuel subjected to combustion in a combustion chamber, converts a pressure produced through the combustion into a mechanical work, and outputs the converted mechanical work.
For protection of the global environment, reducing the emission of air pollutants from the internal combustion engine is highly demanded. Another strong demand is further reduction of the fuel consumption, in order to lower the emission of carbon dioxide as a cause of global warming and reduce the driving cost of the internal combustion engine.
One measure to meet these requirements is application of a combustion system that compresses and auto-ignites an air-fuel mixture in a combustion chamber (in the specification hereof, this combustion system is referred to as ‘premix compression ignition combustion system’) to the internal combustion engine. As described later in detail, the internal combustion engine adopting the premix compression ignition combustion system is expected to simultaneously and significantly reduce the emission of the air pollutants included in the exhaust gas and the fuel consumption. This combustion system compresses and auto-ignites the air-fuel mixture. Under some driving conditions of the internal combustion engine, the timing of auto-igniting the air-fuel mixture is too early, and the air-fuel mixture is undesirably auto-ignited in the course of compression. This leads to the occurrence of severe knocking.
One proposed technique forms an air-fuel mixture having a gradually decreasing concentration of a fuel in the combustion chamber and ignites part of the air-fuel mixture having a greater concentration of the fuel, so as to control the timing of auto ignition of the remaining air-fuel mixture (PATENT LAID-OPEN GAZETTE No. 2001-254660). This proposed technique ignites and combusts the part of the air-fuel mixture, so as to raise the internal pressure of the combustion chamber and compress and auto-ignite the remaining part of the air-fuel mixture. A delay time to auto ignition of the compressed air-fuel mixture (auto ignition delay time) is lengthened with a decrease in concentration of the fuel. The compressed remaining air-fuel mixture is thus not auto-ignited all at once, but auto ignition starts successively from part of the remaining air-fuel mixture having a greater concentration of the fuel. Regulating the ignition timing of the air-fuel mixture controls the timing of starting the successive auto ignition of the air-fuel mixture and prevents the occurrence of knocking.
Such prior art technique, however, has difficulties in reliably controlling the timing of auto ignition of the air-fuel mixture and in preventing the occurrence of knocking. The fuel is to be mixed with the air to produce the air-fuel mixture, prior to ignition. A certain time period is required for mixing the fuel with the air between injection of the fuel into the combustion chamber and actual ignition of the air-fuel mixture. Under some driving conditions of the internal combustion engine, however, the air-fuel mixture is auto-ignited before the fuel and the air are sufficiently mixed with each other. In such cases, the air-fuel mixture is auto-ignited prior to ignition. Namely the timing of auto ignition of the air-fuel mixture is not controllable by regulating the ignition timing.
The decrease in concentration of the fuel in the whole combustion chamber naturally lengthens the auto ignition delay time and prevents auto ignition of the air-fuel mixture prior to ignition. The excessively low concentration of the fuel, however, makes it hard to ignite the air-fuel mixture.